LEDs 1 + 2 lit, fan spinning but nothing more

My late 2015 iMac was running a 3.3GHz i5-6600 and a SATA SSD (had previously been fusion drive which failed so I removed the SSD and upgraded the SATA HDD to a SATA SSD - this was several years ago).

I decided to give it a bit of an upgrade an go for the i7-6700k 4.0GHz processor and move the SSD from SATA to NVMe using a "M.2 NVMe SSD Adapter" + Crucial P3 2TB SSD M.2 PCIe 3.0 NVMe. Amazon reviewers had successfully used this combo in an iMac.

I installed the new components, powered up and nothing happened - no diag LEDs, nothing. I moved back to the old CPU and still the same.

I assumed this was a PSU problem and replaced it - still the same.

I then moved to the system board, got a new one, transferred my 6700k, the NVMe and tried again - same result.

I then removed the NMVe and discovered that now I get the LEDs 1 & 2 illuminating and the CPU fan spins, however, I can't get anything else.

I've now got 2 system boards that do exactly the same - LEDs 1 & 2 lit, fan spinning but nothing more. The LED status would suggest a GPU issue, however, this seems unlikely - both GPUs on the 2 system boards were working fine before I started messing with the CPUs and NVMEe.

I've tried all different combinations of new CPU, old CPU, RAM, SATA SSD attached, SATA SSD not attached, removing WIfi modules, old PSU, new PSU - everything.

I've tried NVRAM resets etc but I'm not convinced the keyboard / USB ports are getting power (USB optical mouse does not light up).

I have turned the machine on whilst holding the power button in an attempt to reset the SMC but also to no avail.

Could I have a dodgy CPU which has damaged the system boards / PSU. Is it possible that my NMVe adaptor has damaged both system boards and/or the PSUs? I'm reluctant to buy even more components as this 'minor upgrade' is starting to cost more than an new machine - needless to say, I wish I'd never started!.

Are here any details available as to what the voltages on the PSU ribbon cable should be?

What would the symptoms of a dead CPU be? What LEDs would I expect to get illuminated with a dead CPU? Any further suggestions much appreciated.